When servicing and repairing heavy vehicles, it is possible to, when necessary, mechanically release the parking brake of the vehicle. Heavy vehicles are normally equipped with pneumatically activated brake systems, where the brakes when driving thus are activated by means of compressed air. In conventional brake systems, the parking brake is activated by letting the compressed air out of the system, thereby causing a pressure spring to actuate a piston in a brake cylinder for activation of the brake to the parking brake position. Without compressed air in the brake system the parking brake is thus normally activated.
In an earlier, common type of brake cylinder, it is possible to mechanically release the parking brake of the vehicle during service and repair of the vehicle, by unscrewing a release screw in the rear gable of the brake cylinder. This release screw, by means of a head at its one end, brings the piston with it and thereby compresses the pressure spring so that the parking brake is released and put out of function. From the safety point of view, it is of the utmost importance that service and repair personnel can carry out a visual control of the brake cylinder, in order to unambiguously determine whether the parking brake is activated or has been mechanically released, and is thus not activated. In this case, the visual control is done simply by checking whether the release screw is in its screwed out position or not, i.e. if it significantly protrudes from the gable of the brake cylinder. In this kind of brake cylinder a clear indication is thus obtained as to whether the parking brake is activated or not.
The above described protruding release screw however, has the disadvantage of demanding a relatively large free space adjacent to the gable of the brake cylinder, in order to be screwed out to its full length. Since the space available is extremely limited in the area around the wheel suspension in chassis of modern transport vehicles, another known solution is often used, where the release screw instead is axially locked in the brake cylinder and the piston is brought out by a rotationally locked, moving nut which has been screwed on to the release screw. The advantage of this solution is that the release screw stays within the brake cylinder, and thus only a screw head for the manipulation of the release screw is located outside of the brake cylinder during the entire release sequence. A serious safety problem with such a solution however, is that no external and visually observable indication of the activation status of the parking brake (i.e. function/non-function) is obtained. This fact can for example cause service personnel to mistakenly think that the parking brake is fully activated, when it in reality might be only partially activated, and in the process of letting go which might lead to unexpected and undesired movement of the vehicle with a risk for serious injuries to personnel and surrounding equipment.